The rotor slot conductors of squirrel cage rotor electric machines must be electrically insulated from the steel lamination stack of the rotor to prevent stray electrical currents from leaking into the lamination stack. Such currents cause loss of efficiency and an unbalanced magnetic pull in conventional induction machines. The insulation system that is employed for such machines must withstand subsequent manufacturing operations, such as the die casting of cast rotors or the insertion of fabricated bars within the slots, and the welding or brazing of end rings on the fabricated rotors. In addition, the insulation system that is employed must also withstand the thermal, electrical, and centrifugal stresses that occur during operation of the electrical machine.
In brushless doubly-fed induction machines, cage rotor bar voltages during normal operation can be much higher than in conventional induction machines, and these machines therefore require better insulation than conventional induction machines for successful operation. Other effects must also be taken into consideration in doubly-fed electrical induction machines. For example, the parasitic torque effects of doubly-fed rotor leakage are described by Koch and Spee in their paper entitled, “A Comparison of Stack Preparation Methods for Bar Insulation in Die Cast Rotors,” pp. 182-187, presented at the IEEE Industry Applications Society Annual Meeting, Oct. 5-9, 1997.
Prior Art Die-Cast Rotors and Insulation Systems
FIG. 1A shows a typical (prior art) die-cast squirrel cage rotor 100 for a conventional induction machine employing a stack of laminations 101 with individual laminations 110, which are shown in FIG. 2 (prior art). Laminations 110 are stacked with slots 111 either aligned axially or aligned with a small skew relative to the axial direction. End rings 102, as shown in FIG. 1C on rotor 100 typically include fan blades, as shown, and often include other elements, such as balancing posts (not shown). The end rings and bars 103 are integrally die cast of aluminum to form a complete rotor conductor structure. Slot insulation 104, which as shown in FIG. 1B, is disposed between each cast bar 103 and lamination stack 101, may or may not be included in standard induction machines. Laminations 110 are each typically oxide coated to insulate the laminations from each other. Insulation systems for die-cast rotors may include an oxide coating on the laminations applied after punching the laminations from sheet stock, or an iron fluoride insulation coating, which is produced by a sulfur hexafluoride atmosphere treatment. An iron oxide slurry core-plating of the lamination stack may also be employed, as described in the above reference. U.S. Pat. No. 5,488,984 describes a procedure for applying insulation to the slots of rotor laminations using sodium tetraborate decahydrate and sodium nitrite. However, none of these prior art insulation systems provides ideal insulation for high efficiency and minimum unbalanced pull, and none enables an effective consumable pattern casting of the conductors with copper. Other prior art insulating systems for fabricated rotors either employ slot liners or use an insulating film that is applied to conductive bars that are then inserted into the slots; however, both of these approaches are expensive.
A prior art refractory insulation system for electrical coils is disclosed in U.S. Pat. No. 5,634,434. This insulation system is applied to wire during or just prior to winding of the wire into a coil. No reference is made to applying a refractory coating to any laminations comprising the core of the coil.
Casting Methods Applicable to Producing Rotors
The two most common consumable pattern casting methods are lost wax and lost foam. The lost wax method is widely used in jewelry and art casting and for casting difficult materials, such as titanium, which is sometimes used in turbine engine rotors. Because of its accuracy, the lost wax method is commonly used to produce complex parts with very close tolerances. The typical procedure is to produce a pattern of wax that is identical to the desired finished part, except that the pattern is made a small percentage oversize, to allow for the shrinkage of the cast metal during cooling. Gates, which are usually formed as short, small-diameter rods of wax, are added to the wax pattern to connect a sprue to the pattern, usually at the lowest points of the pattern. The sprue, also usually made of wax, defines the opening and path through which molten material is channeled into a mold and typically has a relatively large cross-sectional area to enable the molten casting metal to be carried from a funnel-shaped port at the top of the sprue, into the pattern area through the gates. Vents, formed either using small rods of wax or tubes, are added to the pattern at the high points and other locations, to provide a path for gases to escape during casting. The complete assembly, including pattern, gates, and sprue, is then coated with several layers of a refractory, usually applied by spraying or dipping in a water-solution, ceramic slurry that is allowed to air dry between coats. The ceramic is removed from the ends of the vents of the coated assembly, and the assembly is heated to melt away the wax and strengthen the ceramic coating, resulting in a preheated ceramic mold having the pattern cavity of the part to be cast. This mold is placed in a vents-up position while still hot from preheating, and molten metal is poured into the sprue cavity through the funnel-shaped port to fill the pattern cavity from the bottom up. Gases in the pattern cavity escape through the vents to enable complete filling of the pattern cavity. The metal in the gates freezes (solidifies) before the metal in the sprue, to prevent molten metal from the pattern cavity being pulled back into the sprue during the cooling process. After cooling, the ceramic mold is broken away from the casting and the metal of the gates, sprue and vents is removed, yielding the desired part.
An alternative to lost wax casting is lost foam casting. The lost foam casting technique is an industrial process that is particularly useful for producing complex shapes, such as aluminum engine blocks and cast iron electric motor shells. This process uses a consumable pattern, gates, and sprue—all formed of expanded polystyrene foam or other type of plastic foam. The foam pattern is typically formed by casting separate pieces of the finished assembly with expanded foam and gluing these pieces of foam together to form the complete consumable foam assembly. The foam assembly is then dipped into a water solution slurry containing a suspended refractory material. This slurry coats the pattern and leaves a thin heat-resistant, gas permeable layer after the coated assembly air dries. The coated foam is placed in a flask (i.e., an open top barrel), which is vibrated while sand is added. The sand settles around the coated pattern within the flask and mechanically supports the thin refractory layer before and during casting. Molten metal is then poured into the sprue part of the mold. The heat of the molten metal vaporizes the foam as the molten metal rises in the mold, filling the void that is formed in the mold as the foam vaporizes, to produce a nearly exact replica of the foam pattern. The pattern is again made slightly larger than the desired size of the metal part, to compensate for the shrinkage of the metal that occurs during cooling. The gas permeable refractory coating left behind after the foam has vaporized determines the shape of the part and vents the gases produced by the vaporized foam. Lost foam castings can optionally include metal inserts, which are inserted into the cast foam and supported as necessary before and during casting, so that the metal inserts become an integral part of the cast metal part.
Typically, the die-cast conductive structure of prior art rotors has been formed of aluminum metal, although, it would often be preferable to use copper conductors and end caps, because of the better electrical conductivity of copper compared to aluminum. Die-cast copper conductors have not been used in induction machine rotors because copper is cast at least at a 500-600° F. higher temperature than aluminum. The higher temperature used for making die-cast rotor conductor assemblies of copper places too much thermal stress on portions of the dies, resulting in very short die life. Indeed, even if cast from aluminum or other lower-temperature metal, the rotor conductor assemblies can place too much thermal stress on the conventional oxide coatings used for insulation and thereby cause electrical leakage currents.
Cast induction machine rotors therefore require a more effective rotor lamination stack insulating system to improve efficiency and reduce unbalanced magnetic pull, particularly, if it is desired to use cast copper for the conductors in the slots and for the end caps. Thus, an electrical insulating system that also provides thermal insulation of the lamination stack from the molten metal during casting enables lower pressure die casting or the use of consumable pattern casting with copper to improve efficiency. A more rugged lamination insulating system would also improve the cost effectiveness of fabricated rotors. A better (i.e., lower cost to fabricate and more effective) insulating system is clearly essential if doubly-fed cage rotors are to be feasible in commercial applications, since doubly-fed cage rotors will not operate properly if there are leakage currents through the insulation separating the conductors from the metal used for stack of laminations.